This invention relates to a threaded joint for steel pipes for use in connecting steel pipes such as oil well pipes. More particularly, this invention relates to a threaded joint for steel pipes which has excellent galling resistance and gas tightness and which does not require the application of a compound grease containing a heavy metal powder, which application was conventionally carried out before each time fastening was performed in order to prevent the joint from galling.
Oil well pipes which are steel pipes used in the drilling of oil wells are connected with each other by a threaded joint for steel pipes. The threaded joint is comprised of a pin having a male thread and a box having a female thread.
As schematically shown in FIG. 1, a male thread 3A is normally formed on the outer surface at both ends of a steel pipe A to form a pin 1, and a female thread 3B is formed on both sides of the inner surface of a separate joint member in the form of a sleeve-shaped coupling B to form a box 2. As shown in FIG. 1, the steel pipe A is normally shipped in a state in which a coupling B is previously connected to one end.
A threaded joint for steel pipes is subjected to compound pressures due to axial tensile forces caused by the weight of the steel pipe and the coupling and internal and external pressures underground, and it is also subjected to heat underground. Therefore, a threaded joint is required to maintain gas tightness (sealability) without being damaged even under such conditions. In addition, during the process of lowering oil well pipes, it is often the case that a joint which has once been fastened is loosened (unfastened) and then refastened. Therefore, according to API (American Petroleum Institute), it is desired that there be no occurrence of severe seizing called galling and that gas tightness be maintained even if fastening (make-up) and loosening (break-out) are carried out ten times for joints for tubing and three times for joints for casing.
In recent years, in order to improve gas tightness, special threaded joints which are capable of forming a metal-to-metal seal have come to be generally used. In this type of threaded joint, each of a pin and a box has an unthreaded metal contact portion in addition to a threaded portion having a male or female thread, and both the threaded portion and the unthreaded metal contact portion form a contact surface between the pin and box. The unthreaded metal contact portions of the pin and the box come into intimate contact with each other to form a metal-to-metal seal portion and contribute to an increase in gas tightness.
In such a threaded joint capable of forming a metal-to-metal seal, a lubricating grease with high lubricity called a compound grease has been used in order to prevent the contact surface, particularly the metal contact surface, from galling. This grease, which is a kind of liquid lubricant, is applied to the contact surface of at least one of the pin and the box prior to fastening. However, this grease contains a large amount of harmful heavy metal powders. When the grease which is squeezed out to the periphery during fastening is cleaned with a cleaning agent, the compound grease and the used cleaning agent flow out into the ocean or the soil and cause environmental pollution, and this has come to be considered a problem. In addition, there was the problem that the application of grease and cleaning which were repeated before each fastening lowered the working efficiency in the field.
As threaded joints for steel pipes which do not need the application of a compound grease, JP 08-103724A, JP 08-233163A, JP 08-233164A, and JP 09-72467A disclose threaded joints in which a solid lubricating coating comprising a resin as a binder and molybdenum disulfide or tungsten disulfide as a solid lubricant is applied to a threaded portion and an unthreaded metal contact portion (namely, to the contact surface) of at least one of a pin and a box.
In these Japanese patent publications, in order to increase the adhesion between the solid lubricating coating and the substrate steel, it is disclosed to form, as an undercoating layer for the solid lubricating coating, a manganese phosphate chemical conversion coating layer or a combination of a nitride layer and a manganese phosphate chemical conversion coating layer, or to provide the contact surface with surface irregularities having an Rmax of 5-40 xcexcm.
In JP 08-103724A, it is disclosed to use, as a lubricating powder, a molybdenum disulfide powder having a particle diameter of 0.45-10 xcexcm and preferably 2-5 xcexcm as measured by the Fischer method. With a particle diameter of less than 0.45 xcexcm, no effect on improving against galling is obtained, while a 10 xcexcm or higher particle size gives no additional effect on improving lubricity and makes it difficult to adjust the thickness of a solid lubricating coating.
It might be expected that the use of a threaded joint in which the contact surface of a pin and a box has a solid lubricating coating to provide lubricity thereto would make it possible to dispense with the application of a compound grease and thus avoid the aforementioned problems regarding the environment and working efficiency.
However, with a conventional solid lubricating coating, it is not possible to attain a high anti-seizing effect such as can be obtained by application of a compound grease, and a seizing flaw called galling occurs after fastening and loosening are repeated less than 10 times. Thus, there was the problem that it was not possible to prevent galling in a stable manner and maintain gas tightness.
Recently, a heat-resistant threaded joint for steel pipes has been desired for use in high-temperature oil wells in which the temperature reaches 250-300xc2x0 C. which is higher than the temperature in conventional oil wells, or in steam injection oil wells into which steam at a high temperature close to the critical temperature (e.g., around 350xc2x0 C.) is injected in order to improve oil recovery. Therefore, there are cases in which it is required for a threaded joint to guarantee galling resistance and gas tightness when a joint which has been fastened is subjected to a heating test at a temperature of 250xc2x0 C. or higher and then subjected to loosening and re-fastening.
A conventional threaded joint having a solid lubricating coating formed on the contact surface in fact had an extremely inferior galling resistance compared to the case where a compound grease was applied, particularly when exposed to a high-temperature environment.
Thus, with a conventional threaded joint in which a solid lubricating coating is formed, it is still necessary to apply a compound grease, and the aforementioned problems with respect to the environment and working efficiency cannot be eliminated.
It is an object of this invention to provide a threaded joint for steel pipes having a solid lubricating coating which is capable of maintaining galling resistance and gas tightness in a stable manner without application of a compound grease.
It is another object of this invention to provide a threaded joint for steel pipes having improved galling resistance, which can prevent the occurrence of galling and a decrease in gas tightness upon repeated fastening and loosening without application of a compound grease when used in drilling for crude oil in high-temperature environments such as in deep, high-temperature oil wells or steam injection oil wells.
The present inventors investigated why a difference in performance between solid lubricating coatings occurs while concentrating on the structures of solid is lubricating coatings. As a result, it was found that the resistance to galling of a solid lubricating coating in a repeated fastening-loosening test is governed by the state of distribution (form of aggregates) of a lubricating powder present in the coating rather than by the particle diameter of the lubricating powder itself as described in JP 08-103724A.
Namely, when most particles of a lubricating powder in a solid lubricating coating are aggregated such that they are present in the form of large masses of aggregates or secondary particles having a size of 15-60 xcexcm in equivalent circular diameter (equivalent diameter of an equal area circle) which is defined below, stable galling resistance can be ensured.
In addition, it was also found that the occurrence of galling which is particularly significant when fastening and loosening are repeated in a high-temperature environment is because a solid lubricating coating abrades soon due to its decreased abrasion resistance at a high temperature and that the high-temperature abrasion resistance of a solid lubricating coating can be markedly increased by incorporating a fibrous filler in the coating.
In one embodiment, the present invention is a threaded joint for steel pipes comprising a pin and a box each having a contact surface including a threaded portion and an unthreaded metal contact portion, characterized in that the contact surface of at least one of the pin and the box has a solid lubricating coating comprising a lubricating powder and a binder, wherein the proportion of area of a cross section along the thickness of the solid lubricating coating which is occupied by secondary particles of the lubricating powder having an equivalent circular diameter of 15-60 xcexcm is from 5% to 90%.
In the present invention, the particle diameter of a secondary particle means the diameter of a powder aggregate (secondary particle) present in a solid lubricating coating. The equivalent circular diameter of a secondary particle of a lubricating powder will be described below.
In another embodiment, the present invention is a threaded joint for steel pipes comprising a pin and a box each having a contact surface including a threaded portion and an unthreaded metal contact portion, characterized in that the contact surface of at least one of the pin and the box has a solid lubricating coating comprising a lubricating powder, a fibrous filler, and a binder in which the mass ratio of the fibrous filler to the binder is in the range of from 0.01 to 0.5.
In a preferred embodiment of the present invention, the lubricating powder is one or more powdery substances selected from molybdenum disulfide, tungsten disulfide, organomolybdenum compounds, graphite, boron nitride, and polytetrafluoroethylene.